This invention relates to new and improved oil compositions, and more particularly it relates to oil compositions prepared from synthetic ester lubricating oils.
Deterioration of lubricating oils and formation of harmful deposits under high performance service conditions is a major problem in providing satisfactory lubrication of engines which are subjected to high temperatures such as internal combustion engines. This problem is especially acute in lubricating engines of the gas turbine and jet engine type, e.g., turbojet, turboprop, or prop-jet, turbofan, and supersonic jet transport engines, where lubricants encounter extremely high temperatures. These severe service conditions, which favor oxidation reactions, can break a lubricant down into harmful acidic products which attack the metal and rubber parts of the engine or which may act as further oxidation catalysts.
The early development of the gas turbine was carried out using relatively conventional mineral oil lubricants. With further development of this engine, it became clear that mineral oils were unable to satisfy the demands placed on the lubricant by the extremes of temperature that occur within these high performance engines. Oxidation instability at high temperature and inadequate viscosity index were two of the major disadvantages of mineral oil lubricants. Attempts were made to incorporate additives into the mineral oil lubricants to improve their performance. For example, viscosity index improvers, thickeners, or other highly non-volatile additives were incorporated in mineral oil lubricants to improve their performance, but these additives proved to be undesirable for use in such engines because of their tendency to leave a residue which accumulates and interferes with the operation of the engines.
The prior art has discovered that lubricants based on synthetic ester oils perform far more satisfactorily than mineral oils in meeting the conditions encountered in jet engines, but because of the severe operating conditions, such ester base oils, without further compounding also tend to rapidly decompose, corrode metals and vital engine parts, and attack synthetic rubber oil seals even though improved metal alloys and oil seals are now conventionally employed.
The problem of attack of rubber oil seals is especially acute with oil seals made of silicone elastomers. Nearly all engines and/or accessories made today, both of the commercial and military type, have some silicone seals which come in contact with the lubricant. Thus, an oil in time may cause cracks in the seal resulting in oil loss and other related problems. Further, lubricants used in high performance jet engines are forced to bear a high load. The lubricating art is filled with myriads of corrosion inhibitors and load carrying improvers. Unfortunately, many of the load-carrying additives which have been used in synthetic ester oil lubricants have a damaging effect on silicone seals.
To overcome these problems, the prior art has continued to search for new additives which can be incorporated into the synthetic ester oils to improve their properties. For example, as disclosed in U.S. Pat. No. 3,720,612, which was granted on Mar. 13, 1973 and which is assigned to the same assignee as the present application, trihydrocarbyl neutral phosphate ester additives and amine neutralized addition salts of one or more acidic phosphate esters of between about 14 and about 60 carbon atoms per salt molecule are incorporated in synthetic ester oils to provide the ester oils with improved load carrying properties and to reduce their propensity to attack silicone elastomer seals. Because of the constant upgrading of the performance requirements of turbo engines, a continuing search, however, goes on for additives having still better properties.